How about we just implement a system that when a vehicle brakes hard it also send out a low power directional signal (to the rear) that reads "Hard Braking, #1 vehicle, ".
Then every vehicle that receives it replies with "Hard Braking, #2 vehicle, " and every vehicle that receives it replies with "Hard Braking, #3 vehicle, ", etc. Then at some predetermined cutoff point (number dependant on the vehicle's speed) the vehicles stop propagating the message.
The point of the random number is so that your vehicle can ignore multiple receipts of the same braking event while not identifying the vehicle.
That should cover the vast majority if situations that you want your vehicle to warn you about.